1. Field of the Invention
This invention is principally in the field of internal combustion engines, and particularly internal combustion engines for burning solid fuels, such as coal or coke, as well as liquid and gas fuels. This invention is also related to the field of external combustion, Stirling cycle, engines.
2. Description of the Prior Art
Our national dependence on imported petroleum aggravates our trade imbalance and creates a wartime vulnerability to blockade. Greater use of domestic coal, particularly in transportation engines, could reduce both the vulnerability and the trade imbalance. Domestic coal reserves, as well as worldwide coal reserves, are much larger than petroleum or natural gas reserves. Additionally, coal is much less costly, per unit of energy, than natural gas or petroleum derived fuels. It has thus been recognized for some time that it would be very much in the national interest to have available an efficient and durable transportation engine, capable of operating on coal, or coal derived, solid fuels. Recent prior art efforts for this purpose have been largely directed along the following lines:
A. Slurries of coal particles suspended in water have been successfully and efficiently burned in essentially conventional diesel engines. But the resulting ash particles caused increased wear of engine cylinders and piston rings. Aggravated wear also occurred on the fuel injector nozzles. As a result, this development work has been discontinued. A summary of some of these developments is presented in the following reference: xe2x80x9cCoal Fueled Diesel Engines, 1993,xe2x80x9d J. A. Caton and H. A. Webb, Editors, ASME Publ. No. ICE-Vol. 19, 1993.
B. Experiments using coal and coke in chunk form, on a fixed fuel bed, have been carried out for combustion in piston internal combustion engines. A two-stage combustion of coal gasification, followed by gas burnup, is described in my U.S. Pat. No. 4,412,511, Nov. 1, 1983. A single stage combustion is described in my U.S. Pat. No. 5,479,893, Jan. 2, 1996, and U.S. Pat. No. 5,485,812, Jan. 23, 1996. Abrasive ash particulates, carried over out of the fixed bed, being larger than slurry fuel particulates, would be capable of causing even greater cylinder and piston ring wear in conventional engines.
C. The Stirling cycle engine, originally developed by the Rev. Robert Stirling in the early 1800s, is an external combustion engine, capable of using almost any fuel, including coal. Recent developments of Stirling cycle engines, as substitutes for conventional internal combustion engines, are described in the following references:
(1) xe2x80x9cStirling Engines,xe2x80x9d G. T. Reader and C. Hooper, E. and F. N. Spon, New York, 1983.
(2) xe2x80x9cStirling Engines,xe2x80x9d G. Walker, Clarendon Press, Oxford, 1980; To achieve competitive fuel efficiency, and engine size, required the use of expensive superalloy materials, for the heat exchangers, and special seals, to contain the working fluids at very high pressures. As a result, these recently developed Stirling Cycle engines have failed to replace conventional internal combustion engines.
D. Internal combustion Stirling engines, using mechanisms essentially similar to those of external combustion Stirling cycle engines, have been developed, in various forms, during the past one hundred years. These internal combustion Stirling engines were developed starting in the late nineteenth century, in an effort to meet the competition of the conventional internal combustion engine, then undergoing development. Recent examples of these internal combustion Stirling engines are described by Thring, U.S. Pat. No. 5,499,605, and Webber, U.S. Pat. No. 4,630,447. These prior art internal combustion Stirling engines utilized fuels which did not create abrasive wear particles, and were not capable of durable operation on ash producing fuels, such as coal. The greater mechanical complexity of the Stirling engine mechanism, as compared to conventional internal combustion engines, prevented these prior art internal combustion Stirling engines from successfully competing against conventional internal combustion engines.
E. Our national energy independence could thus be greatly assisted if an efficient and durable coal burning engine were available, which avoided the abrasive wear problems due to using coal in conventional internal combustion engines, and avoided the high cost problems, due to superalloys and special seals in external combustion Stirling cycle engines.
The term, xe2x80x9cStirling Cycle Engine,xe2x80x9d is used herein to refer to external combustion engines, operating on variations of the Stirling cycle invented by the Rev. Robert Stirling.
The term xe2x80x9cinternal combustion engine,xe2x80x9d is used herein, and in the claims, to refer to an engine wherein fuel and air are mixed together, and then ignited and burned to combustion products, and these gases act directly upon the work producing engine piston, to create the engine power output.
The term, xe2x80x9cSwept volume,xe2x80x9d is used herein and in the claims to mean the volume added to or subtracted from the volume adjacent to the crown of a piston moving within a cylinder.
The term, xe2x80x9cliquid fuel,xe2x80x9d is used herein and in the claims to include an homogeneous liquid fuel, such as gasoline or diesel fuel; a slurry of immiscible liquid fuel in water or other liquid, such as residual petroleum fuel suspended in water; a slurry of solid fuel suspended in a liquid, such as coal in water slurry fuel.
The term xe2x80x9csolid fuel in chunksxe2x80x9d is used herein and in the claims to refer to solid fuel, such as coal or coke, in chunks large enough that they cannot be suspended in a liquid slurry.
A multifuel internal combustion Stirling engine of this invention uses two pistons, a displacer piston, and a compressor piston, reciprocating within a common cylinder, to carry out a power producing cycle.
Air enters the cylinder between the pistons, is compressed therebetween, is burned with an engine fuel while being displaced out of the volume between the pistons, through a combustion chamber and into a burned gas volume on the opposite side of the displacer piston. The burned gases are then expanded, producing a work output greater than the work input of compression. Following expansion, the burned gases are discarded and fresh air introduced into the cylinder between the pistons, for the next following cycle.
The mechanical components of this internal combustion Stirling engine are somewhat similar to the mechanical components of prior art Stirling cycle engines. But a multifuel internal combustion Stirling engine of this invention differs from a Stirling cycle engine, in that fuel is burned with the working fluid air inside the engine, and the burned gas discarded with each cycle, whereas the working fluid of a Stirling cycle engine is retained within the engine, through all of the repeated cycles, and is alternately heated and cooled to create a net work output. In a Stirling cycle engine the working fluid is not burned with a fuel, combustion occurring external to the engine, where the working fluid is to be heated.
To obtain adequate fuel efficiency from prior art external combustion, Stirling cycle engines requires the use of expensive superalloy materials for the heat exchangers, where the working fluid is heated. A principal beneficial object of the multifuel internal combustion Stirling engine of this invention is that internal combustion is used instead of a heat exchanger and these expensive superalloy heat exchangers are avoided.
When coal or other solid fuels are burned in conventional prior art internal combustion engines, the resulting abrasive ash particles cause severe engine wear and consequently increased engine maintenance costs. In a multifuel internal combustion Stirling engine of this invention, the burned gases containing the abrasive ash particles are retained within the burned gas volume and do not contact that portion of the cylinder over which the compressor piston moves. In this way ash particle wear of the engine is largely avoided, since the displacer piston is not pressure loaded and does not require use of gas tight seals, such as piston rings. This is another beneficial object of the multifuel internal combustion Stirling engines of this invention, that coal fuels and coal water slurry fuels can be used without excessive engine wear.
Widespread use of these multifuel internal combustion Stirling engines would introduce price and supply competition between the many different kinds of fossil fuels, such as coal, petroleum and natural gas. This economic competition is a clear way to energy independence, since coal reserves greatly exceed petroleum and natural gas reserves, both nationally and internationally.